Reductive dimerization of acrylonitrile

ABSTRACT

A PROCESS FOR THE REDUCTIVE DIMERIZATION OF A,B-UNSATURATED NITRILES, AMIDES OR ESTERS SUSCEPTIBLE TO MICHAEL REACTION BY TREATMENT WITH ALKALI METAL OR ALKALINE EARTH METAL AMALGAMS IN THE PRESENCE OF COMPOUNDS WHICH ARE PROTON DONORS BUT DO NOT REACT APPRECIABLY UNDER THE REACTION CONDITIONS, PARTICULARLY WATER, AND IN THE PRESENCE OF HEXAMETHYLETHYLENETETRAMINE.

United States Patent U.S. or. 260-4658 4 Claims ABSTRACT OF THEDISCLOSURE A process for the reductive dimerization of afi-unsaturatednitriles, amides or esters susceptible to Michael reaction by treatmentwith alkali metal or alkaline earth metal amalgams in the presence ofcompounds which are proton donors but do not react appreciably under thereaction conditions, particularly water, and in the presence ofhexamethylethylenetetramine.

This invention relates to the reductive dimerization of a,fi-olefinicnitriles, carboxylic amides and carboxylic esters, particularly ofacrylonitrile.

It is known from a number of patent specifications, for example U.S,patent specification No. 3,193,574 and British patent specification No.1,063,497, that oafl-olefinic compounds, which are susceptible to thereaction known as Michael reaction (cf. Organic Reactions, vol. X, P ges179-555), for example acrylonitrile, acrylamide or acrylic esters, canbe dimerized by treatment with alkali metal or alkaline earth metalamalgams in the presence of proton donors. These methods are as a rulecarried out in a neutral to weakly alkaline medium, for example in thepH range of from 7.0.to 9.5. Inert solvents may also be used. It ispreferred to carry out the methods using basic catalysts suchasquaternary ammonium salts, unsubstituted or N-substituted amides,lactams, sulphoxides, sulphones, sulphonates or ureas. In these priorart methods good yields of dimeric compounds with reference to reacteda,B-olefinic compounds are obtained with relatively low conversion ratesor only low space-time yields are achieved.

It is an object of the invention to provide a process for the reductivedimerization of u,B-Olefinically unsaturated nitriles, carboxylic amidesor carboxylic esters susceptible to Michael reaction in which processhigh conversion rates and high yields and space-time yields are acieved.

It is another object of the invention to provide a process for reductivedimerization in which easily accessible compounds are used as additives.

We have found that a,/3-olefinic nitriles, amides or esters which aresusceptible to Michael reaction can be reductively dimerized bytreatment with alkali metal or alkaline earth metal amalgams in thepresence of proton donors which do not react appreciably with theolefinic compound under the reaction conditions, particularly water, ata high rate of conversion, with good yields and good space-time yieldsby carrying out the reaction in the presence of hexamethylenetetramine.

Particularly suitable a,fl-olefinic nitriles, amides or esters which aresusceptible to the Michael reaction are acrylonitrile, acrylamide andacrylic esters of low molec ular weight alcohols, for example thosehaving one to four carbon atoms, and also alkyl derivatives of the saidcompounds, for example those having one to three carbon atoms in thealkyl radical such as methyl, ethyl, propyl or isopropyl derivatives.Examples of other suitable olefinically unsaturated compounds apart fromthose already specified are for example crotononitrile, crotonamide, andthe corresponding methacrylic derivatives,, such as methacrylonitrile,methacrylamide or methacrylic esters.

The most important application of the process is in the reductivedimerization of acrylonitrile to adiponitrile.

Alkali metal amalgams such as sodium, potassium, lithium, rubidium orcaesium amalgams are generally used, but the process may be carried outjust as well with alkaline earth metal amalgams such as magnesium,calcium 0r barium amalgams. Easily accessible sodium amalgams ispreferred.

The content of alkali metal or alkaline earth metal in the amalgam mayvary within wide limits: it is preferred to use amalgams having acontent of up to 0.8%, particularly from 0.01 to 0.4%, by weight ofalkali metal or alkaline earth metal, it is however possible to useamalgams having a higher content than 0.8% by weight.

Water is particularly suitable for use as the proton donor. It is alsopossible however to use lower alcohols, alone or mixed with water,provided they do not react appreciably with acrylonitrile under thereaction conditions, for example lower alcohols having one to threecarbon atoms such as methanol, ethanol or isopropanol.

The process is generally carried out by placing in a vessel the0:,[3-Ol6fiIliC compound, the hexamethylenetetramine (which may also beprepared in situ from ammonia and formaldehyde) and the proton donor andthen slowly adding the amalgam. Since reaction of the ama1- garn takesplace very rapidly, large amounts of amalgam do not accumulate. Thea,fl-olefinic compound and the proton donor are as a rule used in themolar ratios which lie within the limits 5:1 to 1:6, particularly 2:1 to1:3. Hexamethylenetetramine is advantageously used in an amount of from0.05 to 1.2 moles, preferably from 0.1

to 0.5 mole, with reference to 1 mole of a, 8-o1efinic compound. Theamalgam is usually supplied in an amount of from 0.01 to 0.8, preferablyfrom 0.07 to 0.3, gram-equivalent of alkali metal or alkaline earthmetal in the amalgam, with reference to 1 mole of a,[i-olefiniccompound.

Itis advantageous to maintain a temperature of about 0 to 50 C. (forreasons of cooling preferably from 25 to 35 C.) in the reaction mixture.Furthermore it is advantageous to maintain a pH value of 7 to 10 in thereaction mixture, advantageously by adding polybasic acids, such asphosphoric acid, boric acid or sulphuric acid. It is also possiblehowever to pass in hydrogen chloride or preferably carbon dioxide. It isadvantageous to introduce carbon dioxide at a slightly superatmosphericpressure. When only the a,;3-olefinic compound, hexamethylenetetramineand proton donors are used, the reaction mixture is often inhomogeneous.Provision therefore has to made for good mixing to ensure thoroughdispersion. It is also possible to provide for improved dispersion orfor homogeneity of the solution however by adding inert solvents actingas promoters. Example of suitable solvents are ethers which are misciblewith water, such as dioxane or tetrahydrofuran, or polyhydric alcoholssuch as glycol or alkyl nitriles (cf. US. patent specification No.3,356,708), sulphones or ,sulphoxides (cf. French patent specificationNo. 1,467,091), or N alkyl substituted alkane carboxylic acid amidessuch as dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone(cf. the published papers of Dutch patent applications Nos. 6609103 and6610715). Other additives which act as promoters, for example alkyl oraryl sulphonates or the corresponding free acids (of. British patentspecification No. 1,069,326) may also be used. The relative proportionsused in the cited patent specification may be used. The content ofu,;9-olefinic com-pounds is generally more than 10% by weight withreference to the reaction mixture.

It is advantageous to add a conventional stabilizer for thea,;8-olefinically unsaturated compound against undesired polymerization,for example hydroquinone or p-nitrosodimethylaniline. Amounts of from0.1 to 1 p.p.m. with reference to the reaction mixture are generallyused.

The reaction mixture may be worked up for example by separating mercury,filtering off any bicarbonate precipitated or dissolving it by addingwater and then recovering the dimerization product. Since the process ispreferably carried out in a two-phase system, it is sufficient toseparate the organic phase and to fractionate it. The aqueous phase maybe immediately reused. When the reaction has been carried out in thepresence of a solvent in which a homogeneous phase forms, it isadvantageous to extract the dimerized product with a nonpolar organicsolvent such as methylene chloride or toluene and then to separate theorganic phase by distillation.

The invention is illustrated by the following examples. The parts areparts by weight unless othrewise stated.

EXAMPLE 1 5000 parts of sodium amalgam (with 0.28% by weight of sodium)is added within twenty minutes to 400 parts of a solution (consisting of40% by weight of acrylonitrile, by weight of hexamethylenetetramine, 20%by weight of water and 20% by weight of isopropanol) laced in acontainer provided with a thermometer, cooler, glass electrode for pHmeasurement, inlet pipe for carbon dioxide and a stirrer, while coolingwith ice water and with vigorous mixing. The pH value in the reactionmixture is kept at 8.5 to 9.5 by passing in carbon dioxide and thetemperature is regulated at to C. When all the amalgam has been added,the whole is stirred for a short time and then the mercury is separated,800 parts of water is added to the solution and it is extracted oncewith 200 parts of toluene and then again with 150 parts of toluene. Theorganic phase is distilled. A yield of adiponitrile of 82% by Weightwith reference to acrylonitrile and a yield of 67% by weight withreference to amalgam is determined by gas chromatographic analysis.

EXAMPLE 2 with reference to acrylonitrile and with reference to sodiumin the amalgam.

EXAMPLE 3 The procedure of Example 2 is followed but a mixture is usedwhich consists of 40% by weight of acrylonitrile, 30% by weight ofhexamethylenetetramine and 30% by weight of water. 3500 parts of sodiumamalgam with 0.4% by weight of sodium is added to this emulsion withintwenty-five minutes. The pH value is kept at from 8.5 to 9 by means ofcarbon dioxide and the temperature. is kept at 35 C. When the reactionis over, mercury is separated and precipitated bicarbonate is filtered01f and washed with a little water. The aqueous phase is separated fromthe organic phase and the latter is washed with a little water to removehexamethylenetetramine and is then distilled. Adiponitrile is obtainedin a yield of 84% with reference to acrylonitrile and of with referenceto amalgam.

We claim:

1. Improvement of the process for the reductive dimerization ofacrylonitrile to form adiponitrile by treating it with alkali metal oralkaline earth metal amalgams at a temperature of from 0 to 50 C. and ata pH value of from 7 to 10 in the presence of a proton donor which doesnot react appreciably with the olefinic compound under the reactionconditions, which improvement comprises carrying out the reaction in thepresence of from 0.05 to 1.2 moles of hexamethylenetetramine per mole ofacrylonitrile.

2. A process as claimed in claim 1in which the amalgam used is sodiumamalgam.

3. A process as claimed in claim 1 in which the molar ratio of theacrylonitrile in the proton donor is from 5:1 to 1:6.

4. A process as claimed in claim 1 wherein the said amount is 0.1 to 0.5mole per mole of acrylonitrile.

References Cited UNITED STATES PATENTS 3,193,574 7/1965 Katchalsky etal. 260-4658 JOSEPH P. BRUSTQPrimary Examiner US. Cl. X.R. 260-485, 561

